In paper and textile industries and also in other fields of industry, a moving web material is dried. In the production and finishing of paper, there are a number of stages at which drying has to be carried out by means of a method not contacting the web, for example by drying by means of radiation.
The infrared radiator devices currently used for drying of a web material consist of high-temperature quartz-tube radiators or of gas-operated medium-wave radiators. The wavelength range of a high-temperature short-wave radiator is substantially 0.5 . . . 5.0 .mu.m, while the peak is at about 1.2 .mu.m. When a thin web is dried, the short-wave radiation penetrates through the web, because the absorption coefficient of the material is, as a rule, poor in the wavelength range between 0.5 .mu.m and 2.0 .mu.m, as the absorption peak is in a range substantially higher than 2 .mu.m. Thus, the emission peak of the radiator and the absorption peak of the web material do not coincide. However, with a high-temperature short-wave radiator, a high power density per unit of area is achieved. The power density may be up to 450 kW per sq m, in which case the radiation energy absorbed into the web is higher than 130 kW per sq m. Power densities of said order are required in an attempt to obtain quick drying, which is again necessary, for example, in a process of coating of paper.
The wavelength range of medium-wave infrared radiators is substantially 1.5 .mu.m . . . 6.0 .mu.m. The wavelength corresponding to the maximum intensity is placed approximately between 2.0 .mu.m and 3.0 .mu.m. One of the points of absorption maximum of the water to be evaporated is situated within said interval. At said interval, the absorptivity of cellulosic fibres is also good. Out of the reasons mentioned above, the radiation efficiency of the radiation of a medium-wave radiator is high, about 40-60%, whereas the corresponding efficiency with short-wave infrared radiators, i.e. with a high-temperature radiator, is about 30-35% when drying of thin web materials is concerned. When the thickness of the material is increased, the efficiency of absorption becomes higher especially for the short-wave radiators.
The maximum power density attainable with medium-wave infrared radiators is 60 . . . 75 kW per sq m when a one-sided source of radiation is used, and 120 . . . 150 kW per sq m when a two-sided source of radiation is used.
A dryer composed of an infrared radiator device, i.e. an IR-dryer, consists of a radiation face, which is placed as close to the face to be dried as possible. In the prior-art devices, the radiation face is enclosed in a box, and the box is fixed in a suitable location on the frame constructions of the- process equipment either stationarily or as provided with a displacing mechanism. Further, in said dryers, the use of a backup reflector is known, which reflects the radiation that has passed through the material to be dried and thereby intensifies the process of drying.
From the prior art, a number of different IR-dryers used for drying of a moving web or web material are known. The operation of these dryers is based on the ability of pieces to emit electromagnetic radiation, which is specific of the temperature of the piece. It is a second feature characteristic of radiation that, instead of one wave-length, the radiator emits several wavelengths, whereby an emission spectrum specific of the radiator is formed. Further, in accordance with the laws of physics, it is characteristic of radiation that, when the temperature of the radiating piece rises, the transfer of radiation heat to the target material is increased in proportion to the difference between the fourth powers of the temperatures of the pieces.
However, the temperature of the radiator does not alone determine how much radiation can be absorbed into the material to be dried. The temperature, moisture, thickness, material, surface roughness, and brightness of the piece to be dried determine an absorption coefficient, which indicates what a proportion of the radiation arriving on the face of the piece to be dried is absorbed into the material. However, as a rule, the absorption coefficient is a function of the wavelength, so that in a short-wave range the absorption coefficient of a thin material is inferior to that in a medium-wave or long-wave range.
IR-radiation sources operating in the short-wave infrared range are considered radiators which emit a radiation whose wavelength of maximum intensity of radiation is in the wavelength range of 0.76 . . . 2.00 .mu.m. IR-radiation sources operating in the medium-wave infrared range are considered radiators which emit a radiation whose wavelength of maximum intensity is in the wavelength range of 2.00 . . . 4.00 .mu.m.
The correspondence with temperature is obtained by means of Wien's displacement law from the formula EQU .lambda..sub.maximum .times.T=2.8978.multidot.10.sup.-3 (mK)
The temperature range of a short-wave radiator is obtained as 3540.degree. C. . . . 1176.degree. C., and that of a medium-wave radiator as 1176.degree. C. . . . 450.degree. C.
The IR-dryers operating in the short-wave range are currently almost exclusively electrically operated. In them, usually a tungsten filament placed in a quartz tube is made to glow by means of electric current. The maximum emitter temperature of the glowing filament is usually about 2200.degree. C., in which case the wavelength corresponding to the maximum intensity of radiation is about 1.2 .mu.m.
In the prior-art short-wave infrared radiators, the lamps are, as a rule, arranged in heating modules of 3 . . . 12 lamps. The modules are attached side by side, and a drying zone extending across the web is obtained. The lamps are usually spaced so that the power density of the dryer per unit of area varies in a range of 100 . . . 450 kW per sq m.
The dryers operating in the medium-wave IR range are either electrically operated or gas-operated. In electric devices, filaments are made to glow by means of electric current either in a quartz tube or behind a ceramic tile or a tile made of quartz. In the former case, the spiral filament operates directly as the emitter, whereas in the latter case the heat is transferred first into the tile, after which the tile operates as the emitter. The tile may also be partly penetrable by radiation. In gas-operated systems, a usually ceramic radiator is made to glow by means of a flame, which radiator starts glowing and thus operates as the emitter. Radiation is partly also emitted directly from the flame. As was stated above, the wavelength of maximum intensity of medium-wave infrared radiators is 2.00 . . . 3.00 .mu.m, the corresponding temperature of the radiator being, as was stated above, in the range of 1176.degree. C. . . . 690.degree. C. With medium-wave infrared radiators, the maximum power density varies, depending on the method and the temperature, substantially in a range of 40 . . . 100 kW per sq m.
Adverse aspects of short-wave infrared radiators include-poor radiation efficiency in the shorter wavelength range of the radiator influencing the overall efficiency, expensive electric control system, high cost for electricity and ventilation systems.
Adverse aspects of medium-wave infrared radiators include low power density per unit area when quick drying is aimed at, poor adjustability, slow heating and cooling, relatively high cost of electrical system and electricity in the case of electric infrared radiators. For gas operated systems the high cost for the gas feed system and the risk of explosion from handling of explosive gases can be mentioned.
The difficulties to use the cooling exhaust air or the exhaust gases for an efficient improvement of the drying process is common for both gas- and electrical medium wave dryers.
Thus, it can be considered that a major drawback of the prior art infrared heaters, ie. IR-dryers, consisting of short wave infrared radiators is poor efficiency because of the low absorption coefficient of the material to be dried in the shorter wave length range of the radiator.
When the IR-dryer consists of medium wave infrared radiators, a particular drawback can be considered to be the low power density and still the need for a relatively expensive electrical and ventilation system, poor controllability because of the slow heating and cooling of the medium-wave radiators and the difficulties to efficiently use the exhaust air or gases in the drying process.
In the EP Patent 288,524, a method is described for drying a moving web material.
In the method, infrared radiation is directed at the material to be dried, and the moving web material is passed through the radiation zone of the infrared radiator while the web material to be dried absorbs radiation into itself. In the method, the radiation produced by at least one first infrared radiator and the radiation produced by at least one second infrared radiator are directed at the moving web material to be dried, said radiators being fitted in the vicinity of one another. In this connection, the wavelength of the maximum intensity of the radiation of the first infrared radiator is shorter than the wavelength of the maximum intensity of the radiation of the second infrared radiator, in which case, in the drying process, the spectrum of the overall radiation is optimal in view of the absorption spectrum of the material to be dried. The maximum intensity of the radiation of the first infrared radiator occurs in the wavelength range of the radiation 0.76 .mu.m&lt;.lambda..sub.maximum &lt;2.00 .mu.m, and the maximum intensity of the radiation of the second radiator is in the wavelength range 2.00 .mu.m&lt;.lambda..sub.maximum &lt;4.00 .mu.m. The radiators can be fitted at the same side of the moving web material, or they can be fitted so that the first radiator is placed at one side of the web material and the second radiator at the opposite side.
By means of the method and the device in accordance with the EP Patent 288,524, a spectrum is obtained that is favourable in view of the drying. Then, an efficiency of radiation is achieved that is at least about 5% better than with the prior-art solutions of equipment.
From the prior art, it is known to provide the second radiator, placed at the opposite side of the web material to be dried, with a surface layer which in the short wave 0,5-2,0 .mu.m spectra mainly reflects but partly also absorbs the radiation of the first infrared radiator that passes through the material web so that the temperature of the second infrared radiator rises to several hundreds of Celsius degrees. When a typical white ceramic material is used as the surface material, the temperature may rise to a value of an order of 500 . . . 700.degree. C. for low grammage webs for example paper webs with grammages less than 110 g/m.sup.2. A temperature of 500 . . . 700.degree. C. is not yet sufficient as the surface temperature of the second infrared radiator, while its power density is a function of its temperature level in Kelvin degree in fourth power, but additional electric energy can be fed into the surface layer of the second infrared radiator according to EP Patent 288,524, whereby the surface temperature can be raised further to a temperature of 800 . . . 1050.degree. C.
Thus, the backup radiator described above is a device that receives the heat radiation passing through the web and uses this heat for heating the surface layer of the device. The backup radiator is a medium-wave radiator. The backup radiator is used together with a short-wave infrared radiator. Together, these two devices produce a good drying result and efficiency.